Magnitude and trend of NOx emissions constrained by OMI observations and associated impacts on O3 and human health

Accurate estimates of magnitudes and trends of emissions of nitrogen oxides (NO_x) are important for understanding formation of air pollutants and the effectiveness of emission control strategies. We estimate long-term global (2°x2.5° resolution, 2005 - 2017) and regional (0.5° x 0.67° resolution, 2005-2012) NO_x emissions for East Asia and North America using a newly derived hybrid mass balance / 4D-Var inversion framework by assimilating OMI NO₂ observations into the GEOS-Chem adjoint model. Our top-down NO_x emissions in China increase from 2005 to 2011 and start to decrease from 8.0 TgN/year in 2011 to 7.0 TgN/year in 2015, whereas the reduction of top-down NO_x emissions in the US has slowed since 2010. No clear NO_x emission trend is observed in Western Europe and Japan. Summertime O₃ is lower than that estimated using bottom-up anthropogenic emissions (from HTAPv2) in most US regions, reducing model bias compared to surface O₃ data. Annual mean O₃ generally has lower magnitude than estimates from bottom-up emissions in East Asia, and the coastal US, but have higher magnitude in the mid-US. The GEOS-Chem simulated O₃ trends using our top-down NO_x emissions are consistent with ambient O₃ measurements reported previously - from 2005 to 2012, simulated summertime mean O₃ concentrations in China increase by 0.5 ppbv although decreases occur around populated and industrial regions that are dominated by NO_x titration; simulated summertime O₃ has increased from 2005 to 2012 in the Western US but decreased in the Eastern US. We further apply concentration - response functions to calculate the relative risk of mortality from ischaemic heart disease, chronic obstructive pulmonary disease, lung cancer and stroke associated with O₃ pollution. We then separate the contributions to trends in these health effects driven by NO_x emissions vs meteorology, thereby quantifying how remote-sensing based emission estimates refine understanding of air pollution health impacts.